This invention relates to polyorganosiloxane compositions. More particularly, it relates to addition curable polyorganosiloxanes and still more particularly to materials having improved adhesion to plastic substrates.
The use of polyorganosiloxanes (hereinafter sometimes "silicones") in the form of elastomeric compositions is now widespread. Elastomeric silicones include room temperature vulcanizable compositions, which cure upon standing in the presence of moisture, and addition curable compositions, which cure rapidly upon heating.
Addition curable silicones are of particular use when rapid cure is desirable or when the presence of moisture or, ultimately, of the by-products formed during the curing of room temperature vulcanizable materials is not desired. Illustrative uses are as potting compounds, in sealed environments in which water is not tolerated or in electronic applications. Curing of the addition curable silicones may be by direct heating or by an equivalent means such as ultrasound.
Cured silicone elastomers prepared from addition curable compositions are frequently lacking in adhesion, especially to plastic substrates. Thus, improvement of their adhesion to plastics is very desirable.
U.S. Pat. No. 5,164,461 discloses the use of bis(3-methoxysilylalkyl) fumarate and maleate esters as adhesion promoters for addition curable silicones on both metal and plastic surfaces, described generically. It also describes the curing of such silicones at temperatures from 100.degree. to 150.degree. C. It has been found, however, that compositions containing these esters are difficult to cure at lower temperatures. Since substantially lower curing temperatures, frequently no higher than 85.degree. C., are required in the case of many resinous and especially thermoplastic substrates to avoid heat-induced distortion, there is a need for alternative adhesion promoters which are effective for lower temperature curing after application of the silicones to plastic substrates.
The present invention includes a class of adhesion promoting esters for addition curable silicones which, when incorporated in said silicones, are effective with plastic substrates and at low curing temperatures.